1. Field of the Invention
The present invention relates to an ultrasonic imaging technique for differentiating the distribution of scatterers within a tissue, especially to an ultrasonic imaging technique for differentiating the distribution of scatterers within a tissue, which utilize Nakagami parameter m and has a correcting and an imaging procedure.
2. Description of the Prior Art
The grayscale ultrasound image system (B-mode) is a frequently used tool to noninvasively examine the tissue anatomy in the medical diagnosis. The grayscales in the B-mode image are determined according to the strengths of the echoes from the changes in acoustic impedance in the tissues. However, the use of the B-mode image in the clinical diagnosis might suffer from some disadvantages.
First, to more clearly visualize the structures in a tissue, the typical ultrasonic scanner allows operators to adjust different system parameters, such as the system gain, time-gain compensation (TGC), and dynamic range. Moreover, different ultrasonic scanners made by different manufacturers have different procedures of signal and image processing. It means that the B-mode image is easily affected by the system factors.
Second, the specular reflection is highly angle-dependent. When the sound beam is perpendicular to the interface of the tissue, the transducer can receive a large amount of the returned acoustic energy. Otherwise, only less energy is received. It implies that the brightness of the B-mode image also rely on the skill and training of the operators.
Third, during imaging, the scattering will occur when the incident wavelength is greater or comparable to the dimension of the scatterers in a tissue. The generated backscattered signals would form the so-called speckle, which often exhibits a granular pattern of white and dark spots in the ultrasonic B-mode image.
To avoid the influence of the speckle effect on the image quality, many methods were proposed to reduce the speckle appearance in the B-mode image. Nevertheless, due to that the backscattered signals are actually dependent on the shape, size, density, and other properties of the scatterers in a tissue, the information related to the scatterers carried by both the backscattered echoes and other weak signals might be lost in the B-mode image.
To resolve the dilemmas of the ultrasonic B-scans, different kinds of quantitative methods that are independent of the effects from the systems and operators are developed to provide the information of the scatterers, which may be associated with the nature of the biological tissues, for assisting in the diagnosis of the B-mode image. Considering the randomness of the ultrasonic backscattered signals, many researchers apply various statistical distributions to model the shape of the probability density function (pdf) of the backscattered echoes for the tissue characterization. The Rayleigh distribution is the first model used to describe the statistics of the ultrasonic backscattered signals. The pdf of the backscattered envelope would follow the Rayleigh distribution when the resolution cell of the ultrasonic transducer contains a large number of randomly distributed scatterers. It should be noted that the scatterers in most biological tissues have various possibilities of arrangements. If the resolution cell contains the scatterers that have randomly varying scattering cross sections with a comparatively high degree of variance, the envelope statistics are the pre-Rayleigh distributions. If the resolution cell contains the periodically located scatterers in addition to the randomly distributed scatterers, the envelope statistics are the post-Rayleigh distributions. And therefore some useful distributions including Rician, K, homodyned K, and generalized K are applied to encompass both the pre-Rayleigh and post-Rayleigh statistics of the backscattered envelope. However, the computational complexity of these models in their parameter estimations may limit their practical applications.
Several years ago, the Nakagami distribution, initially proposed to describe the statistics of the radar echoes, was applied to the statistical analysis of the ultrasonic backscattered signals. The Nakagami statistical model has comparatively less computational complexity and is general enough to describe a wide range of the scattering conditions in medical ultrasound, including pre-Rayleigh, Rayleigh, and post-Rayleigh distributions. Although the Nakagami distribution can fit well with the pdf of the ultrasonic envelope, a compounding statistical distribution may be more appropriate to model the envelope statistics, because the ultrasonic signals returned from the tissues may contain contributions from more than one mechanism. Hence, the compounding Nakagami distributions involving the Nakagami-Gamma, Nakagami-lognormal, and Nakagami-inverse Gaussian were subsequently developed to fit more closely to the envelope statistics of the ultrasonic backscattered echoes from tissues. In these distributions, the primary parameter to determine the backscattered statistics is the Nakagami parameter, which is estimated from the statistical moments of the ultrasonic backscattered envelope. It only depends on the shape of the backscattered envelope and has been demonstrated using computer simulations and experiments on phantoms to have a good ability to differentiate different scatterer concentrations in a medium. The Nakagami parameter has also been applied to practical measurements on biological tissues to classify the scatterer properties, such as the bone, skin, breast, and blood.
As a result of the outstanding ability of the Nakagami parameter to detect the variation of the scatterer concentration, this parameter was suggested to form the parametric image in making a medicine diagnosis, and afterward some preliminary studies and applications have been performed. However, there is no complete report related to the Nakagami parametric image to date.